This invention relates to an improved voltammetric apparatus of the Static Mercury Drop Electrode (hereinafter SMDE) type, in which the mercury is purified and recycled
Electrochemical detector and voltammetric cells are known in the art and have been used with success for the analysis of trace elements in the laboratory. Two-electrode and three-electrode cells are known. The three-electrode cell comprises a working electrode, a counter-electrode and a reference electrode which has the function of establishing and maintaining a constant potential relative to the working electrode or the sample solution. In principle, the electrodes may be affected by poisoning due to absorption with resulting passivation and loss of signal. In order to avoid such poisoning, the dropping mercury electrode has been adopted in many such cells.
U.S. Pat. No. 3,922,205 describes the basic structure of a polarographic cell. U.S. Pat. No. 4,138,322 discloses a structure of shielded dropping mercury cathode. U.S. Pat. No. 4,260,467 describes a dropping mercury electrode which comprises a reservoir for liquid mercury, a mercury capillary at the outlet end of which mercury drops are formed, and a valve for selective air-purging passage of mercury from the reservoir to the inlet end of the capillary. An automated polarographic cell is described by C. N. Yarnitzky in Analytical Chemistry, Vol. 57, No. 9, August 1985, p. 2011-2015.
Such cells, however, are not fully satisfactory. In some cases, they include solid electrodes which becomes polluted with time. Others are complicated and unreliable or require a very large volume of the sample solution. In others the mercury feed apparatus is complicated, and mercury has to be replaced once a while.
An improved voltammetric apparatus, free from said drawbacks, is disclosed and claimed in PCT application WO 96/35117. It comprises:
a) a cell body housing, in addition to a reference electrode, a working electrode and, in its lowermost portion, a counter-electrode;
b) means for removing oxygen from the sample solution;
c) means for feeding the sample solution to said deoxygenation means, means for feeding a stream of an inert gas to said deoxygenation means, and means for causing said solution to flow in said deoxygenation means, whereby oxygen is removed therefrom by contact with said inert gas;
d) a means for removing said inert gas from said deoxygenation means after deoxygenation of the sample solution;
e) an inlet for the deoxygenated sample solution provided in said cell body in the space between said working electrode and said counter-electrode;
f) an exit for the sample solution provided in said cell body at a level above said working electrode; and
g) vacuum and/or pressure means for causing said sample solution to flow to said exit, to be discharged from the cell above said working electrode, thus assuring that the space between said working electrode and said counter-electrode is constantly filled with said sample solution.
Still, the use of mercury drop electrodes, while beneficial in many respects, involves health and ecological problems, from which even the aforesaid improved voltammetric cell is not free. The operator, who feeds mercury to the cell, comes into contact with it. The mercury, which has formed the drops, collects in a sump, which must be handled to recover it. The mercury drop forms at the lower end of a capillary tube and this latter becomes clogged at comparatively frequent intervals, so that it must be replaced. In order to replace the capillary tube, the mercury must be removed from the mercury reservoir. In all these operations and manipulations, the operator comes, to a greater or smaller extent, into contact with the mercury, which contact is ecologically negative and involves a health hazard. These drawbacks are, of course, common to the mercury drop voltammetric cells of the prior art, and this invention has the purpose of eliminating them in any cell in which they exist.
Further, prior art voltammetric apparatus are not satisfactory for carrying out for anodic stripping techniques. Therein, the mercury drop remains in place for a time from 3 to 15 seconds, depending to the capillary used. While this lifetime of the drop is sufficient for polarography, it is not sufficient for anodic stripping, which requires a much longer drop lifetime, in the order of minutes, e.g. about 2 minutes. Further, prior art apparatus are sensitive to small particles, e.g. in the range of 25 to 100 xcexcm, which can block the capillary tube.
It is therefore an object of this invention to provide an electroanalytical voltammetric apparatus of the Static Mercury Drop Electrode (SMDE) type, which is free of the said drawbacks.
It is another object of the invention to provide such an apparatus, which comprises means for purifying the mercury in situ and feeding the purified mercury back to the capillary tube which contains it and from which the electrode drops are formed, by means which avoid all manipulation on the operator""s part and all contact between him and the mercury.
It is a further object of the invention to provide such an apparatus in which clogging incidents are reduced and which comprises means that enables the use of improved electroanalytic techniques such as anodic stripping techniques.
It is a still further object of the invention to provide such an apparatus in which the capillary tube, at the lower end of which the mercury drop forms, can be replaced, in case of clogging, without the operator""s coming into contact with the mercury.
Other objects and advantages of the invention will appear as the description proceeds.
The electroanalytical voltammetric apparatus according to the invention, comprises, in combination with a SMDE voltammetric cell, means for puriying the mercury and recycling the purified mercury to the capillary tube at the lower end of which the mercury drops are formed.
Said means for purifying the mercury is a means for generating surface contact between the contaminated mercury and water having a high oxygen content. Preferably, the water is saturated or nearly saturated with oxygen or air, and its oxygen content is close to 8 mg/L or higher.
Accordingly, an aspect of this invention is a process for continuously purifying and recycling mercury in an SMDE cell, which comprises continuously bringing contaminated mercury and highly oxygenated water into mutual surface contact, whereby the contaminating metals are oxidized and migrate from the mercury to the water, and continuously feeding the resulting purified mercury to the SMDE cell.
Another aspect of the invention is an apparatus for continuously purifying and recycling mercury in an SMDE cell, which comprises means for continuously bringing contaminated mercury and highly oxygenated water into mutual surface contact, whereby the contaminating metals are oxidized and migrate from the mercury to the water, and means for continuously feeding the resulting purified mercury to the SMDE cell.
Preferably, said puriying apparatus comprises a container hereinafter, the xe2x80x9cpurification containerxe2x80x9d) for continuously receiving contaminated mercury, said mercury accumulating in said container to form a mass having an upper surface, means for forming a layer of highly oxygenated water in said container above said mercury mass, said layer having a lower surface in contact with said upper surface of said mercury mass, and means for continuously withdrawing purified mercury from said container.
Said highly oxygenated water can be produced in any suitable way. A preferred way of producing it consists in forming a layer of water, e.g. salty water, in surface contact with contaminated mercury, and enriching said layer with oxygen. This may be conveniently done, e.g., by introducing contaminated mercury into a purification container, introducing water above the mercury surface to form a layer, and bubbling through said water layer oxygen or an oxygen containing gas, preferably air. Another way of producing said highly oxygenated water layer is to oxygenate water, e.g., by bubbling through it oxygen or an oxygen containing gas, preferably air, or by mixing a water stream with a stream of oxygen or an oxygen containing gas, preferably air, while said water is out of contact with contaminated mercury, and bringing the resulting, highly oxygenated water into contact with the contaminated mercury. This may be conveniently done, e.g., by continuously introducing contaminated mercury into a purification container, continuously introducing the highly oxygenated water into said purification container above the mercury surface to form a layer in contact with the surface of said mercury, and continuously withdrawing said water from said container, whereby to replace the water of said layer with freshly oxygenated water, at such a rate as to maintain therein the desired oxygen content and to limit its contamination to acceptable levels. Preferably, said SMDE voltammetric cell is basically the cell described in said PCT application WO 96/35117 as well as in PCT application WO 96/35118, with which the mercury purification means of this invention are combined. In this case, said inlet into the cell is the inlet into the deoxygenating means. However, this invention can be carried out with voltammetric cells other than those described in said PCT applications, particularly cells which do not include deoxygenation means.